Develop the associative map example.

eg/assoc.castile

@@ -1,36 +1,70 @@
+/*
+ * Implementation of an associative map in Castile.
+ *
+ * The map is implemented as an association list,
+ * but this fact is hidden from clients, as only
+ * the operations have access to the internals
+ * of the struct.
+ */
+
 struct assoc {
   key: string;
   value: string;
   next: assoc|void;
-} for (singleton, update, lookup, remove)
+} for (update, lookup, remove, render)
 
-fun singleton(k: string, v: string) {
-  make assoc(key:k, value:v, next:null as assoc|void)
+fun empty() {
+  return null as assoc|void
 }
 
-fun update(k: string, v: string, a: assoc) {
+fun update(k: string, v: string, a: assoc|void) {
   make assoc(key:k, value:v, next:a as assoc|void)
 }
 
-lookup : assoc, string -> string|void
-fun lookup(a: assoc, k: string) {
-  if a.key == k {
-    return a.value as string|void
-  }
-  n = a.next
-  typecase n is void {
+lookup : assoc|void, string -> string|void
+fun lookup(a: assoc|void, k: string) {
+  typecase a is void {
     return null as string|void
   }
-  typecase n is assoc {
-    return lookup(n, k) as string|void
+  typecase a is assoc {
+    if a.key == k {
+      return a.value as string|void
+    }
+    return lookup(a.next, k)
+  }
+}
+
+remove : assoc|void, string -> assoc|void
+fun remove(a: assoc|void, k: string) {
+  typecase a is void {
+    return a as assoc|void
+  }
+  typecase a is assoc {
+    if a.key == k {
+      return remove(a.next, k)
+    }
+    return make assoc(key:a.key, value:a.value, next:remove(a.next, k)) as assoc|void
+  }
+}
+
+render : assoc|void -> string
+fun render(a: assoc|void) {
+  typecase a is void {
+    return ""
+  }
+  typecase a is assoc {
+    return concat(a.value, concat(",", render(a.next)))
   }
 }
 
 fun main() {
-  a = update("1", "first", update("2", "second", singleton("3", "third")));
-  r = lookup(a, "2");
-  print("um");
+  a = update("3", "third", empty());
+  a = update("2", "second", a as assoc|void);
+  a = update("1", "first", a as assoc|void);
+  print(render(a as assoc|void))
+  b = remove((a as assoc|void), "2");
+  print(render(b))
+  r = lookup(b, "2");
   typecase r is void { print("NOT FOUND"); }
   typecase r is string { print(r); }
-  print("ya");
 }